1. Field of the Invention
The present invention relates to a charging apparatus and an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a copier, a printer, and a facsimile machine, as an image carrier, a photoreceptor having a photosensitive layer containing a photoconductive substance formed on its surface is used. In this construction, after the surface of the photoreceptor is uniformly charged under application of electric charge, an electrostatic latent image corresponding to image information is formed thereon through various image-formation process steps. Then, this electrostatic latent image is developed into a visible image with use of a developer containing toner that is supplied from a developing section. The visible image is transferred onto a recording material such as paper, and is fixed into place under application of heat and pressure by a fixing roller. In this way, an image is formed on the recording material.
In such an image forming apparatus, a charging apparatus is used for charging the surface of the photoreceptor. In general, the charging apparatus is composed of: a discharging electrode for conducting corona discharge; a grid electrode to which is applied an appropriate voltage, for controlling the amount of charge applied to the surface of the photoreceptor by a charging electrode and thus controlling the charged potential on the surface of the photoreceptor; and a support member for supporting the charging electrode and the grid electrode. As the grid electrode, a wire grid electrode formed of stainless steel, tungsten, or the like, a porous platy grid electrode constructed by creating a large number of through holes in a metal plate formed for example of stainless steel (grid substrate) and the like can be used.
Among the grid electrode as mentioned just above, the wire grid electrode is susceptible to adhesion of contaminants such as toner. Due to the deposition of contaminants, the capability of controlling the charged potential on the surface of the photoreceptor becomes insufficient, thus causing lack of uniformity in the charged potential on the surface of the photoreceptor.
On the other hand, being formed of an iron-based metal material such as stainless steel, the porous platy grid electrode exhibits high durability under normal circumstances. However, the negative side is that the porous platy grid electrode is prone to oxidation in the presence of water content under a high humidity environment, ozone and nitrogen oxide generated in accompaniment with corona discharge during charging operation, and the like. In the long-time use of the porous platy grid electrode, for example, operation under a high humidity environment and contact with ozone and nitrogen oxide are inevitable. Therefore, in the porous platy grid electrode formed of a metal material such as stainless steel, corrosion such as rust occurs due to water content in the air, ozone, nitrogen oxide, and the like, and nitrogen oxide is deposited on the surface thereof, in consequence whereof there results durability deterioration. In addition to that, the capability of controlling the charged potential on the surface of the photoreceptor becomes insufficient, thus causing lack of uniformity in the charged potential on the surface of the photoreceptor. This makes it impossible to constantly impart desired charged potential to the surface of the photoreceptor with stability.
In view of the problems associated with such a grid electrode, for example, Japanese Unexamined Patent Publication JP-A 2006-113531 discloses a charging apparatus characterized in that its porous platy grid electrode has a nickel plating layer containing polytetrafluoroethylene (PTFE) fine particles formed on at least one surface thereof. The charging apparatus disclosed in JP-A 2006-113531 employs a grid electrode constructed by forming, on a surface of a porous platy grid electrode, a PTFE fine particle-containing nickel plating layer (hereafter referred to as “nickel PTFE composite plating layer” unless otherwise specified) by means of electroless plating. The grid electrode having the nickel PTFE composite plating layer has the advantage of being inexpensive compared to a grid electrode having a gold plating layer.
However, in the grid electrode having the nickel PTFE composite plating layer, since the nickel PTFE composite plating layer includes heterogeneous components such as metallic nickel and organic fine particles PTFE, it follows that water content in the air, ozone and nitrogen oxide generated through discharge and the like. find their ways from the interface between nickel and PTFE particles to the surface of the grid electrode. This leads to oxidation of the surface of the grid electrode and thus to corrosion such as rust. As a result, the charged potential-control capability and the durability of the grid electrode still remain insufficient, thus causing lack of uniformity in the charged potential on the surface of the photoreceptor.
Meanwhile, as a discharging electrode for conducting corona discharge, a wire electrode, a metal plate electrode having a plurality of needle-like portions (hereafter referred to as “needle-like electrode”), and the like can be used. Among them, the use of a needle-like electrode is particularly desirable because of its advantages of requiring less number of constituent components, having longer service life, generating less amount of ozone, and suffering little from a break and ensuing malfunction. The needle-like electrode is constructed by performing etching on a metal plate mainly formed for example of an iron-based metal material such as stainless steel thereby to form a plurality of needle-like portions. An iron-based metal material such as stainless steel used as the material for forming the needle-like electrode exhibits high durability, but has a drawback that it is prone to oxidation in the presence of water content under a high humidity environment, ozone, nitrogen oxide and the like generated in accompaniment with corona discharge during charging operation. In the long-time use of the needle-like electrode, for example, operation under a high humidity environment and contact with ozone and nitrogen oxide are inevitable. Therefore, in the needle-like electrode formed of a metal material such as stainless steel, corrosion occurs due to water content in the air, and ozone and nitrogen oxide and the like, in consequence whereof there results durability deterioration. In addition to that, there arises deterioration in the capability of controlling a voltage which is applied to the needle-like electrode to induce corona discharge at the needle-like portions, thus causing lack of uniformity in the charged potential on the surface of the photoreceptor. This makes it impossible to constantly impart desired charged potential to the surface of the photoreceptor with stability. Furthermore, also in the wire electrode, just as is the case with the needle-like electrode, there is a problem to be solved that ozone generated through corona discharge induces rust, corrosion, and the like, which results in lack of uniformity in the charged potential on the surface of the photoreceptor.
In view of the problems associated with such a discharging electrode for conducting corona discharge, for example, Japanese Unexamined Patent Publication JP-A 2006-201488 discloses a charging apparatus characterized in that its discharging electrode has a nickel PTFE composite plating layer formed on at least one surface thereof. In the charging apparatus disclosed in JP-A 2006-201488, the nickel PTFE composite plating layer formed on the surface of the discharging electrode is obtained by means of electroless plating, and the film thickness thereof is set at or above 0.3 μm. Accordingly, as compared with a nickel PTFE composite plating layer obtained by means of commonly-used electrolytic plating under DC current application, the nickel PTFE composite plating layer is dense and hard in layer structure, has less pinholes, exhibits film thickness uniformity even if it is made thin, and provides high adherence with respect to the discharging electrode.
However, in the discharging electrode having the nickel PTFE composite plating layer, since the nickel PTFE composite plating layer includes heterogeneous components such as metallic nickel and organic fine particles PTFE, it follows that water content in the air, ozone and nitrogen oxide and the like find their ways from the interface between nickel and PTFE particles to the surface of the discharging electrode. This leads to oxidation of the surface of the discharging electrode and thus to corrosion such as rust. As a result, the applied voltage-control capability and the durability of the discharging electrode still remain insufficient, thus causing lack of uniformity in the charged potential on the surface of the photoreceptor.